Ni-15Cr alloys having high resistance to stress corrosion cracking in high-temperature and high-pressure water have been conventionally used as materials of a high-temperature and high-pressure vessel typically used in a pressurized water nuclear power plant. However, in order to further improve the resistance to stress corrosion cracking, Ni-base high Cr alloys such as Ni-30Cr alloys have been employed in recent years. In welding of the high-pressure vessel, since the same corrosion resistance as that of a base metal is required, a filler metal containing the same components as those of the base metal is needed.
However, when overlay welding or joint welding is performed using a Ni-30Cr filler metal, micro cracking is easily caused in weld metal stacked through multi-pass welding. This grain boundary cracking is called “ductility-dip cracking of the reheated weld metal”, which is distinguished from solidification cracking caused when a weld metal is solidified, and occurs in a temperature range in which the solidification is completed. The ductility-dip cracking of the reheated weld metal is described below. When a weld metal composed of a high-Cr-content Ni-base alloy containing about 300 or more of Cr is repeatedly subjected to reheating during welding, a coarse Cr carbide is precipitated in a grain boundary and the grain boundary strength, that is, the bonding strength between grains adjacent to each other is decreased. Consequently, if a tensile thermal stress or shearing thermal stress is exerted on the grain boundary during welding, the grain boundary is opened.
In PTL 1, Mn and Nb are added to prevent the ductility-dip cracking of the reheated weld metal. PTL 1 discloses a Ni—Cr—Fe alloy weld metal containing Cr: 27 to 31% by mass, Fe: 6 to 11% by mass, C: 0.01 to 0.04% by mass, Mn: 1.5 to 4.0% by mass, Nb: 1 to 3% by mass, Ta: 3% or less by mass, Nb+Ta: 1 to 3% by mass, Ti: 0.01 to 0.50% by mass, Zr: 0.0003 to 0.02% by mass, B: 0.0005 to 0.004% by mass, Si: less than 0.50% by mass, Al: up to 0.50% by mass, Cu: less than 0.50% by mass, W: less than 1.0% by mass, Mo: less than 1.0% by mass, Co: less than 0.12% by mass, S: less than 0.015% by mass, P: 0.015% or less by mass, and Mg: 0.004 to 0.01% by mass, the balance being Ni and incidental impurities.
PTL 2 discloses an austenite-based weld joint and welding material used for high-temperature equipment such as a boiler and a technology of ensuring corrosion resistance by adding 1 to 5% by mass of Cu. In the technology of PTL 2, the content of Mn added as a deoxidizer is set to be 3.0% or less by mass relative to the total mass of the weld joint or welding material, whereby the formation of an intermetallic compound when the weld joint or welding material is used at high temperature for a long time is suppressed and thus the embrittlement is prevented.
In PTL 3, the contents of Si, Mn, Cu, Nb, W, V, and the like added to a covered electrode are specified to form a weld metal having high weld cracking resistance. PTL 3 also discloses that a nitride such as TiN is produced by actively adding N (0.03 to 0.3% by mass) as an incidental impurity to improve the tensile strength of a weld metal.